I. Field of the Invention
The present invention pertains to over-the-air paging systems, and more particularly, to a "talk-back" or "two-way" paging arrangement by which the user of a portable or mobile pager unit may respond to pages or, alternatively, may initiate telephone calls.
II. Description of the Prior Art
One-way radio or receive-only "paging" systems have become increasingly popular in recent years including various sophisticated schemes for relaying information to the user. The original and best-known paging system generates a signal from a fixed-location transmitter in response to actuation of the transmitter by a telephone call being received. The transmitted signal is received by a mobile receiver or pager unit worn about the person. When the signal is received by the appropriate pager, the pager emits a "beep" thereby alerting the user that there is a message being held for him at a prearranged location or that he is to call a prearranged number. The user must then find an available telephone and call the prearranged location or number whereupon he is given his message.
Some paging systems may employ multiple tones, or multiple-tone pulse signaling to distinguish between several predetermined locations or numbers to be contacted for messages. By way of example, one particular sounding beep may signify that a call to the office is in order, while a different-sounding beep may indicate that the call should be to home, instead. More recent advances in pager technology include liquid-crystal-displays mounted to the exterior of the pager. The liquid-crystal-displays may be remotely loaded by the telephone caller to indicate, for example, the telephone number on which the caller can be reached.
A disadvantage of all such one-way paging systems is that the user of the pager unit cannot immediately speak with the call-initiating party directly. Instead, the user must locate an available telephone and place a call to the initiating party to return the page. A further drawback to a one-way paging system is that the pager cannot be utilized to initiate contact with someone else. Instead, the user must, again, locate an available telephone.
Alternatively, conventional two-way telephone and land-mobile communication services are available. Such services permit two-way communication such as is commonly encountered in telephone conversations. Conventional two-way radio telephone communications, however, require separate transmit and receive frequencies to permit simultaneous communication from the mobile unit to the telephone and from the telephone to the mobile unit. Thus, while such a duplex system permits telephone-like conversation, i.e., the ability to talk simultaneously and to interrupt the other party, it also requires the use of multiple radio channels. In many installations, this is impractical due to frequency congestion and channel assignments. Additionally, duplex systems may cost more than simplex or single channel systems such as are used for conventional one-way paging.
Simplex systems have also been used for two-way radio communication but with some drawbacks. In a typical simplex system, the transmitter and receiver operate at the same nominal frequency. To avoid interference, and other possible communication disruptive problems, the transmitter and receiver should not be energized at the same time. Hence, a keying circuit is typically provided which energizes the transmitter when the telephone party is talking so as to transmit these signals to the mobile unit. While the transmitter is energized, or keyed, the receiver is deenergized. When the telephone party stops talking, the keying circuit deenergizes the transmitter and re-energizes the receiver. While the receiver is energized, any signals transmitted by the mobile unit will be received by the receiver and heard by the telephoning party. One notable drawback is that the simplex two-way radio communications system does not permit both parties to talk at the same time, nor can one party interrupt or interject a comment while the other person is talking (i.e., transmitting).
Systems designed for simplex operation thus require a certain degree of user education from the standpoint that persons accustomed to conventional telephone communications must refrain from speaking until the other party has completed its transmission. Failure to wait out the transmission of the other party will result in the interrupter's message not being heard. This inability to interrupt may also result in channel capturing whereby one party or the other can lock up the channel precluding its use by others.
As discussed, in a typical simplex two-way communication system, the originating caller may be using a telephone to gain access to a base station having a transmitter and a receiver which are adapted for simplex communications with a mobile transceiver. Once the communication link is established, only one or the other party may talk at any given time. When the telephone party talks, for example, the transmitter is energized such that telephone signals (e.g., the caller's voice) can be transmitted to and received by the mobile transceiver. During transmission by the transmitter, the receiver is deenergized. Hence, so long as the telephoning party is talking, no messages can be received at the base station. As a result, for so long as the telephone party is talking, the mobile transceiver user cannot interrupt to terminate the conversation or to interject a comment. In this way, the telephone caller can lock up or capture the channel precluding others from communicating through the base station.
Accordingly, it is an object of the present invention to provide a communication switch circuit for a simplex two-way radio communication system which avoids channel lockup by the telephoning party.
A further objective of the present invention is to provide a communication switch circuit for a simplex two-way radio communication system which will permit a mobile unit such as a pager adapted to transmit to interrupt a telephoning party to interject a comment.
An even further objective of the present invention is to provide a communication switch circuit to permit a transmitting mobile unit to initiate a call to a telephone party.
A yet further objective of the present invention is to provide a communication switch circuit which permits a transmitting mobile unit to terminate the connection between a telephone party and the mobile unit.
It is a further objective of the present invention to provide a communication switch circuit meeting the above objectives and compatible for use in one-way pager systems and/or with duplex radio communication systems.